Conventionally, there are communication systems using time alignment (TA) as a method of controlling the transmission timing in communication terminal stations. Herein, the reason is explained for introducing time alignment to a communication system where TDMA communications are performed, as an example.
A base station that performs TDMA communications receives time slots on which respective channel signals of communication terminal stations are multiplexed through transmission paths. The base station specifies a slot position of each of the communication terminal stations, using a preamble signal added to a data signal of each of the communication terminal stations.
However, when specifying a slot position using a preamble signal, there is such a problem that uplink reception performance is greatly affected by accuracy in detecting slot positions. In other words, an error in the specified slot position degrades the reception performance. Particularly, when a slot position cannot be detected, it is not possible to fetch the data of the slot at all.
Further, when a distance between the base station and a communication terminal station that transmits channel signals is large, a delay (hereinafter referred to as “propagation delay”) occurs until the channel signals reach the base station, and burst signals may collide.
Therefore, the base station detects propagation delays for each channel, and using the detected propagation delays, controls transmission timings in communication terminal stations, thus performing time alignment control. In other words, the base station sets transmission timings for each of the communication terminal stations based on the detected propagation delays for each of the communication terminal stations, and broadcasts set respective transmission timings to the terminal stations as a time alignment control amount, and each of the terminal stations transmits signals according to the time alignment control amount broadcast from the base station. According to such time alignment control, the base station is capable of suppressing amounts of departure from reception timings caused by different distances between the base station and the communication terminal stations and of preventing burst signals from colliding with one another, and thereby the reception quality is improved. Further, since each of the communication terminal stations adjusts the transmission timing according to the time alignment control amount, the base station is capable of predicting the reception timing from the time alignment control amount. Accordingly, even when the base station fails to detect a slot position, the station is capable of receiving the slot.
However, in the aforementioned time alignment control, since a base station needs to broadcast time alignment control amounts to communication terminal stations, there arise problems that a ratio of data transmission burst to a frame decreases and that the frame utility efficiency deteriorates.